1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, printer, facsimile machine and the like, and a control method thereof. More particularly, the present invention relates to an auto cassette changer to automatically change a sheet feeding position from a container unit in current use to another container unit.
2. Description of the Related Art
Recently, there is a need in an image forming apparatus using an electro-photographic process to form an image of high quality on a variety of sheets of plain paper, thick paper, thin paper, OHP and the like (hereinafter, simply referred to as “paper”). To address such need, a method employing an intermediate transfer member has widely been used, wherein images of respective colors of yellow (Y), magenta (M), cyan (C), and black (K) are transferred onto the intermediate transfer member, and an image is formed by overlaying the toner images one on another and transferred as a whole onto a sheet of paper. By using the intermediate transfer member, the transfer onto the paper can be made at one time, which can minimize the influence on transfer efficiency onto the paper so that an image of high quality is obtained.
Meanwhile, in recent years, it has been desirable to adapt the image forming apparatus using the electro-photographic process to the POD (Printing On Demand) market. In the POD market, there is a need for a technique that allows a user to obtain image outputs of a required amount when needed. Thus, technical developments have vigorously been made, based on the electro-photographic process and using the intermediate transfer member, at more rapid and stable formation of an image of better quality.
A conventional image forming apparatus is generally provided with the so-called auto cassette change (ACC) function. According to the auto cassette change (ACC), when a plurality of container units containing sheets of paper are provided having the sheets of the same size set therein, in the event that the container unit in current use becomes empty or attains a paper out state during a continuous copying operation, the container unit is automatically switched to another container unit, while giving a warning to continue the image forming operation.
When the auto cassette change function is employed in the image forming apparatus using the intermediate transfer member, the distance from the point where a latent image is written onto an image bearing member to the point where the images are collectively transferred onto a sheet of paper increases. With the increase in this distance, the time from when the image is written to when it is transferred onto the sheet of paper (image transfer time) increases.
If this image transfer time is longer than the time from initiation of the operation of feeding a sheet of paper from a container unit to the timing when the image is transferred onto the sheet (sheet transfer time), the image will have already been formed on the image bearing member when a sheet is fed from the container unit. That is, at the time when the paper out state within the container unit is detected, image formation has already been started, in which case the sheet feeding operation from a next container unit cannot be made in time.
In such case, if it is possible to check the amount of remaining sheets in the container unit to an accuracy of a single sheet, it would be possible to cause the other container unit to start the sheet feeding operation when the last sheet of paper is fed from the container unit in current use. In this case, the sheet can be fed in time even if the container unit is switched, allowing the image formation and image output to be continued. However, it is difficult to accurately detect the amount of remaining sheets in the container unit on a single sheet basis, for different kinds of paper having various basic weights.
U.S. Pat. No. 6,567,620 proposes an invention to address the problem, wherein when the amount of sheets in a container unit exceeds a prescribed threshold value, the gap between successive sheets is lengthened to lower the productivity level so as to prepare for feeding sheets from a next container unit during that time, and the inter-page gap is narrowed again to resume the normal operation when the feeding of the sheets from the next container unit is started.
Further, Japanese Patent Laid-Open No. 07-121080 proposes another invention to address the problem, wherein when paper exhaustion in a container unit is detected, the sheet feeding operation is stopped, the image formed on the intermediate transfer member is separated from a transfer member while waiting for a sheet fed from a next container unit, and the transfer is carried out after the sheet feeding operation is restarted.
However, each of the inventions described in U.S. Pat. No. 6,567,620 and Japanese Patent Laid-Open No. 07-121080 provides a down time to perform the auto cassette change operation. Since high productivity is required in the POD market, occurrence of lower productivity due to the down time upon the auto cassette change is a problem the user wishes to avoid to the greatest possible extent.